Under Pressure – Drilling for Oil and Gas

Keeping the pressure of an oil or gas rig under control is of paramount importance. Too little pressure and the well may not function correctly, too much pressure and there could be an expensive and dangerous blowout. The wellhead is present to keep the pressure of a production well under control. The wellhead performs this task using a series of valves, spools and adapters, and is situated at the terminal of the wellbore, usually above ground at surface level, welded to the casing strings that stretch far below ground. It provides facilities for casing and tubing hangers, as well as fittings for the Christmas tree and flow control devices.

Due the very dangerous nature of drilling for oil both in terms of the safety of the oil rig workers, and the impact a blowout could have on the environment, production of wellhead components is strictly regulated and each part is tested for potential failure at a microscopic level. There are regulations that must be complied to and these are API 6A Specification for Wellhead and Christmas Tree Equipment and ISO10423 Wellhead and Christmas Tree Equipment. Quality control is the key to the manufacturer of wellhead components, down to the last bolt and nipple. The components must be very strong and must withstand very high pressure levels. Testing in the factories where wellhead components are produced is rigorous. A complete radiographic examination of each part can help expose weaknesses in the casings before they become part of a working wellhead and cause an incident to occur.

Once in place, parts must be checked regularly and replaced when worn. Although wellhead components are manufactured to the highest standards, and much research has gone into creating components for wellheads that will suffer the minimum amount of wear and tear, sometimes there are crevices within the components that simply must be there for the structural integrity of the wellhead as a whole, but that can suffer from corrosion and erosion. A close eye must be kept on any parts that are known to be susceptible to corrosion. They must be checked regularly and the wellhead must be maintained to ensure safety, and continued production.

Many drilling companies will employ maintenance companies to look after their wellheads, performing all the maintenance and repair duties expertly. These professionals generally work solely on wellhead components, so they literally know the wellhead inside out. Regular maintenance will help keep the components in good working order for longer, so it can be a good money saving exercise too, as well as ensuring the well is safe. Parts may need to be replaced or repaired. A maintenance crew will either repair components on site or will take them away to their own facilities to be repaired. 

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Wellhead Christmas Tree

A wellhead christmas tree is the general term used to describe the surface of an oil and gas well. A wellhead Christmas tree provides the structural and pressure-containing interface for oil and gas drilling of production equipment.

The primary function of a well head Christmas tree is to control the flow, usually oil or gas out of the well. A tree can also be used to control the injection of gas or water into a non-producing well in order to enhance production rates of oil from other wells. When the well and facilities are ready to produce or receive oil or gas the tree valves are opened and the formation fluids are allowed to go through a flow line. This leads to a processing facility, storage depot and/or other pipeline eventually leading to a refinery or a distribution centre for gas. Flow lines on subsea wells usually lead to a fixed or floating production platform or to a storage ship or barge, known as a floating storage offloading vessel (FSO) or a floating processing unit (FPU), or floating production and offloading vessel or FPSO.

A tree often provides numerous additional functions including chemical injection points, well intervention means, pressure relief means, monitoring points and connection points for devices such as down hole pressure and temperature transducers.

Tree complexity has increased over the past few decades. They mainly manufactured from blocks of steel containing multiple valves rather than being assembled from individual flanged components. This is especially true in subsea applications where the resemblance to Christmas trees no longer exist given the frame and support systems into which the main value block is integrated.

It's important to remember that a tree and wellhead are separate pieces of equipment which shouldn’t be mistaken as the same piece. The Christmas tree is installed at the top of the wellhead. A wellhead is used without a Christmas tree during drilling operations.

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The Components of an Oil Rig Wellhead

First it is important to understand what a wellhead is. Wellhead is the term given to the part of an oil  or gas well that is above ground. It helps to maintain the structure and pressure of the well drilling equipment beneath it. The wellhead provides the pressure seals and the suspension point for the casing strings. Casing strings are long sections of pipe that run through the whole length of the drill equipment and are cemented into place. Each casing string is typically around 40 feet in length and will have a male and a female connector at either end to allow them to be connected together to reach greater depths. The are fourteen standard casing diameters, from 4.5 inches up to 20 inches. The point of the casing strings is to protect or isolate formations adjacent to the wellbore

When drilling is underway, surface pressure is maintained via a blowout preventer. This prevents a well blowout. This is a massive surge of oil under high pressure. This can occur under water or underground, or above ground. If a blowout occurs above ground the oil can shoot very high into the air as the pressure is so high. This is costly for the rig owner, and can also cause environmental damage. It is dangerous for the workers and it can also destroy expensive equipment. The noise produced by a blowout can make workers go deaf. Oil is also highly flammable so there is a risk of fire too.  So you can see the importance the blowout proventer plays in the drilling process. To find out about our commitment to safety click here.

Once the well has been drilled, a Christmas tree is added tothe top of the wellhead; this has valves and choke equipment to control the flow of fluids through the well. The Christmas tree is so named as it vaguely resembles a decorated Christmas tree in its appearance. However these days, the construction and function of the Christmas tree has become far more complicated, that they no longer really resemble a real Christmas tree, however the name has stuck so it's here to stay. They are used in wells both under the sea and on the surface. They are usually used to control the flow of gas or oil out of the well, but they can also be used to control the flow of gas or water into a well that isn't producing during an attempt to enhance production rates of oil from other wells. The Christmas tree will also serve other purposes. There will be monitoring equipment to keep an eye on various measurements and levels, there will be facilities for adding chemicals to the well, and there will be pressure relief systems built in.

The wellhead is usually welded onto the uppermost casing string, so it becomes integral to the well itself. Wellhead components are expensive, so when the well becomes defunct the components may well be salvaged to be reconditioned and used for other drilling projects.